A circuit substrate in which a circuit pattern is formed on one surface or both surfaces of the circuit substrate is conventionally known (see Patent Literature 1).
The circuit substrate is provided with a solder resist layer to protect the circuit pattern. A letter, a mark or the like to mount electronic parts is displayed on an upper surface of the solder resist layer.
The letter, the mark or the like is formed by forming an ink layer on the solder resist layer, exposing the ink layer by irradiating the ink layer with ultraviolet rays through a film, and heating and hardening ink remaining as a shape portion representing the letter or the like after developing the ink layer.